Several types of non-volatile memory have been developed that rely on resistive memory elements that change resistance under certain conditions. This general category of memory may be referred to as resistive change memory (a.k.a., Resistive Random Access Memory (RRAM)). An RRAM memory element represents stored information as a low resistance state and one or more distinct high resistance states. A dielectric, which is normally insulating, can be made to conduct through the formation of a conducting filament or path. This filament is formed as a result of the application of a sufficiently high voltage (i.e., electric field).
Depending on the type of RRAM memory element, the conduction path formation can arise from different mechanisms. These mechanisms include defect modification, metal migration, ion migration, etc. Forming the filament is generally referred to as “setting” the RRAM memory element. Breaking the filament, usually by applying an appropriately high voltage of an opposite polarity, results in the RRAM memory element resuming a high resistance. This process is generally referred to as “resetting” the RRAM memory element. The RRAM memory element may be “set” and “reset” many times by appropriately applied voltages.
The process of “setting” an RRAM memory element involves applying a sufficiently high voltage (i.e., electric field) across the RRAM memory element. This electric field causes a dramatic reduction in the resistance of the RRAM memory element. This reduction in the resistance of the RRAM memory element remains until the device is “reset” by the application of another sufficiently high voltage—usually of opposite polarity. In some implementations, RRAM memory elements may be paired with an access transistor in an RRAM cell. The access transistor selectively allows current to be passed through, and voltage applied to, the RRAM memory element so that its state may be determined and/or changed. These cells may be arranged in an array so that a particular RRAM cell in the array may be set, reset, or read. Typically, a column of the array is accessed (or controlled) via a conductive path (or wire) that is referred to as a bit line. A row of the array is accessed (or controlled) via a conductive path (or wire) that is referred to a word line.
US Patent Publication No. 2013/0215669 describes a current flowing through the memory element in association with switching the memory element from the high resistance state to the low resistance state. Namely, responsive to the switching, a magnitude of the current flowing through the memory element increases by the ratio of the resistance change. This current flow may cause the filament to grow in size, which can increase a threshold required to switch back to the high resistance state. US Patent Publication No. 2013/0215669, which is herein incorporated by reference in its entirety, describes biasing the select MOS transistor in a 1T-1R memory cell into saturation to act as a current source in order to limit the RRAM element current in a set operation when the element switches from a high resistance state to a low resistance state.